The invention is a duplex filter comprising two ceramic band-pass filters or band-stop filters, or a combination of these filters Both filters have a body of dielectric material with top, bottom and side surfaces, whereby at least the main part of the body is coated with an electrically conductive layer, several holes extending from the top surface to the bottom surface and coated with a conductive material, each hole forming a transmission line resonator, and connecting means for the connection to the resonators.
Radio equipment having both a transmitter and a receiver which use the same antenna, require a transmission network in order to correctly control both the transmission signal and the receiving signal. The signal from the antenna must be directed to the receiver without any substantial interference from the transmitter. Accordingly the signal from the transmitter must be transmitted to the antenna without interference from the receiver. Generally a duplex filter is used for this purpose. It comprises two individual band-pass filters, one being connected in the receiving branch and having a center frequency and a bandwidth corresponding to the receiving band, the other filter being connected in the transmission branch and having a center frequency and a bandwidth corresponding to the transmission band. Often the other ends of the filters are connected through a transmission line to the common antenna line. While the duplex filter often is located in a common housing with interfaces for the transmitter, the receiver and to the antenna, in practice however it will be formed by two individual band-pass filters, because a very high isolation between the filters must be obtained so that their mutual electromagnetic leaks do not interfere with the operation. This is rather easily arranged with filters designed according to the helix technique, because it is possible to place between each filter a metallic partition, which effectively provides the required isolation. On the other hand, heretofore it was not possible to realize a duplex filter made by ceramic techniques in one monolithic ceramic body, because it was not possible to totally avoid the inductive coupling through the ceramic body. In practice it was done so that first the transmission branch band-pass filter and the receiving branch band-pass filter were made separately, each thus having interfaces for the antenna and for the receiver/transmitter. The ceramic bodies of both filters are coated with a conductive layer on the side surfaces and on the bottom surface. The finished filters are soldered onto a common support, that may be a board, a frame, or the like. At the same time the ceramic bodies are mutually fixed by soldering at the end faces. Soldering is possible, because the outer surfaces of the ceramic bodies are coated. The antenna interfaces are joined into one interface, and so a duplex filter is obtained, virtually comprising a single block.
The advantage of the known duplex filter comprising two separate ceramic blocks is a very good isolation between the filter branches, due to the conductive partition between the blocks formed by the coating of each block. On the other hand there is a disadvantage in that the ceramic blocks of each branch must be individually processed, coated and provided with electric connections in order to have the connections to the resonators. Thereafter the finished individual units are mechanically connected. The electrical and mechanical connection of the blocks is a cumbersome and slow operation. In other words, the production capacity must be doubled compared to a situation where it would be possible to make the whole duplex filter in a single ceramic block.
The Finnish patent applications FI-892855 and FI-892856, applicant LK-Products Oy, describe band-pass filters realized in a single ceramic block, where the basis of the inventive idea is that one side surface of the filter is substantially uncoated and that strip conductor patterns are applied on this side surface in order to have the connections to the transmission line resonators. When the circuit patterns are made on the side surface of the body, the filter input and output and the connections between the resonators can be made in a desired way, either purely capacitive or inductive, or as a combination of these. It is also possible to connect block components and inductance wires to the circuit patterns of this side surface, which act on the resonators and on their mutual coupling. This side surface is finally covered with a conductive cover, whereby the ceramic block is entirely enclosed by conductive material.